The objective of the proposed experiments is the understanding of the chemical structure of the metaphase chromosome and its relation to the control of mitosis. The approach concentrates on the function of highly redundant centromeric DNA and the spacer DNA with which it has been associated. Techniques include membrane hybridization, "in situ" hybridization, density gradient sedimentation, renaturation kinetics, and electron microscopy. Materials include Drosophila melanogaster, mouse liver cells, and 3T3 tissue culture cells.